1. Technical Field
This disclosure relates to a developing roller, a developing device, a process cartridge, and an image forming apparatus, and more specifically, to a developing roller that transports developer carried on a developing sleeve to a development area, in which the developing sleeve faces a photoconductive drum across a gap, to develop an electrostatic latent image on the photoconductive drum into a visible toner image, a developing device having the developing roller, and a process cartridge and an image forming apparatus having the developing device.
2. Description of the Background
Image forming apparatuses are used as copiers, facsimile machines, printers, and multi-functional devices combining several of the foregoing capabilities. A conventional type of image forming apparatus carries developer on a developing sleeve of a developing roller to securely transport the developer to a photoconductive drum. The outer surface of such developing sleeve is subjected to surface processing such as sandblasting, grooving, or so-called electromagnetic blasting in which filamentous materials are contacted against the outer surface of the developing sleeve by a rotating magnetic field.
Such sandblasting or grooving may prevent a reduction in image density due to slippage and provide better retention of the developer on the developing sleeve during rotation at high speed.
A conventional type of developing sleeve having an outer surface subjected to sandblasting may be made of aluminum alloy, brass, stainless steel, or conductive resin, for example. Typically, aluminum alloy is used in view of cost reduction and processing accuracy. When performing sandblasting on the outer surface of such developing sleeve made of aluminum alloy, for example, an aluminum tube is extruded in a sleeve shape at high-temperature, and abrasive grains are cold-sprayed against the aluminum tube to form convex and concave portions on the outer surface of the developing sleeve. The surface roughness is in a range of approximately 5.0 μm to 15 μm. Such surface roughening enables the developing sleeve to retain the developer even during rotation at high speed, thereby preventing the developer from slipping.
However, because such convex and concave portions are relatively fine, they may also be abraded by the developer as well as other materials. Accordingly, the outer surface of such sandblasted developing sleeves gradually wears down and becomes smooth as the number of print outputs increases over time. Consequently, a transport amount of developer, which is the amount of developer that the developing sleeve can transport at any given time, may gradually decrease, resulting in such failures as reduced image density. Thus, such conventional sandblasted sleeves suffer from relatively poor durability. It is possible to provide better durability by making the developing sleeve out of a stainless steel having a high rigidity or its outer surface may be otherwise hardened, but at the price of an increase in cost.
A conventional type of developing sleeve having a grooved outer surface may be similarly made of aluminum alloy, brass, stainless steel, or conductive resin, for example. Similar to the above-described developing sleeve subjected to sandblasting, typically such conventional developing sleeve is made of aluminum alloy for cost reduction and processing accuracy. When forming grooves on the outer surface of such developing sleeve made of aluminum alloy, for example, an aluminum tube extruded in a shape of the developing sleeve at high temperature is pulled into cold air and then grooves are formed on the outer surface of the aluminum tube with a die. Typically, such grooves have a rectangular shaped, V-shaped, or U-shaped cross section. Such grooves also have a depth of, for example, approximately 0.2 mm. For example, when such developing sleeve has an outer diameter of 25 mm, typically the number of grooves is approximately 50. Such developing sleeve subjected to grooving, even when rotating at a high speed, is capable of retaining developer in the grooves on the outer surface of the developing sleeve, thereby preventing the developer from slipping on the developing sleeve.
For such grooved developing sleeve, such grooves are relatively larger in size than the convex and concave portions generated by sandblasting and more resistant to abrasion, thereby suppressing a reduction in the transport amount of developer due to a change over time. In other words, such developing sleeve may be more durable than the above-described developing sleeve subjected to sandblasting.
However, in such conventional grooved developing sleeve, the amount of developer transported in the grooves is generally greater than the amount of developer transported in an area having no grooves, thereby resulting in a cyclical variation in image density or so-called “pitch-like uneven density” due to such grooves. Typically, the deeper such grooves, the higher the transport performance of developer while the more likely such pitch-like uneven density is to occur due to, for example, a difference in the intensity of development electric field between the grooves and the lands, or intervals, between the grooves.
By contrast, the shallower such grooves, the less likely such pitch-like uneven density in view of the intensity of the development electric field. However, when the grooves are clogged with toner, additive, and/or carrier, the degree of reduction in the transport performance of developer may increase to such a degree that such pitch-like uneven density occurs more readily.
Hence, in the conventional developing sleeve, the grooves have a depth of not less than 0.05 mm and not more than 0.15 mm to maintain a preferred level of developer transfer performance while preventing occurrence of pitch-like uneven density.
Meanwhile, recent advances in image forming technology, such as a toner and a magnetic carrier of relatively smaller particle diameters or close-proximity developing method, have enhanced image reproducibility, thereby causing such pitch-like uneven density to become more noticeable when it does occur. For example, a development method using a toner having a relatively small average particle diameter of not more than approximately 8.5 μm may provide excellent image reproducibility. At the same time, however, the resultant image is relatively highly sensitive to variation in the amount of developer used for development, thereby causing such pitch-like uneven density to become more noticeable.
A conventional type of image forming apparatus uses a small-particle-diameter toner having a volume average particle diameter of not less than 4 μm and not more than 8.5 μm. In such image forming apparatus, a plurality of grooves is formed on the outer surface of the developing sleeve so as to extend in a longitudinal direction of the developing sleeve. The interval between adjacent grooves is set smaller than the width, in a surface moving direction of the photoconductive drum, of a development area, in which the developer contacts a photoconductive drum, so that the image forming apparatus has at least one groove on the developing sleeve positioned in the development area to prevent the developer carried on the developing sleeve from slipping thereon. As a result, such variation in the amount of developer in such development area may be relatively suppressed compared to an image forming apparatus in which no groove is present in the development area at any given time. Thus, even when using a small particle-diameter toner having a volume average particle diameter of, for example, not more than 8.5 μm, such image forming apparatus may produce a better quality image with excellent image reproducibility while suppressing pitch-like uneven density due to a difference in image density.
However, in the above-described developing sleeve, the interval between grooves must be set relatively small, which may impose a limitation on the method by which the grooves are die-formed after pulling an aluminum tube into cold air. Alternatively, even if the interval between grooves is large enough to accommodate additional grooves, during cutting or grinding performed as finishing the dimension of outer diameter, variations in the depth of grooves may increase, thereby resulting in unevenness in image density.
Meanwhile, with regard to the method for forming grooves, when such grooves are individually cut, the pitch between the grooves can be narrower. Alternatively, when multiple grooves are cut simultaneously, the variation in the depth of grooves can be reduced. However, such methods for forming grooves may increase the number of processing steps, thereby increasing cost.
Alternatively, the above-described electromagnetic blast processing may suppress a reduction in the transport amount of developer due to a change over time. However, because filamentous materials are contacted against the outer surface of a developing sleeve at random, it may be difficult to set a processing condition suitable for providing a long stability of the developer while obtaining an optimal scooped amount of the developer. It may also be difficult to further increase the scooped amount of developer to maintain a high image quality even in a future higher-speed image forming apparatus.
In a conventional type of image forming apparatus, a developing roller may be disposed close to a doctor blade of a plate shape for regulating the thickness of a layer of developer carried on its outer surface to a certain thickness. Typically, the amount of toner supplied to a photoconductive drum is adjustable by adjusting a gap (hereinafter a “doctor gap”) between the doctor blade and the outer surface of the developing roller. Regardless of the shape or surface processing of the outer surface, a friction resistance generated by the developer passing through the doctor gap and a magnetic attraction of the developer may bend the developing roller, thereby causing the doctor gap to be wider at a middle portion in the longitudinal direction of the developing roller than at each end portion supported by a shaft. As a result, the amount of toner supplied is greater at the middle portion in the longitudinal direction of the developing roller than at each end portion, thereby resulting in unevenness in image density in the longitudinal direction of the developing roller.
In view of the above-described situation, the present invention provides a developing roller and a developing device capable of preventing unevenness in image density while suppressing a reduction in the transport amount of developer due to a change over time. The present invention also provides a process cartridge and an image forming apparatus having the developing device.